1. Field of the Invention
The present invention relates to an assist air control system for an internal combustion engine, which controls the supplied amount of assist air to facilitate atomization of fuel injected through a fuel injection valve by opening and closing an assist air passage.
2. Description of the Prior Art
As disclosed in Japanese First (unexamined) Patent Publication No. 1-96457, there has been proposed a so-called timed air mixture system (hereinafter referred to as "timed AMS"), wherein a flow control valve provided in an assist air passage opens and closes synchronously with fuel injected from a fuel injection valve so as to effectively atomize the injected fuel.
In the conventional timed AMS, when the assist air passage remains fully opened due to failure of the flow control valve or its associated control means, assist air continues to be sucked into the engine via the fully-opened assist air passage.
This condition does not cause problems when the throttle valve is significantly opened, since the ratio of assist air relative to the total amount of intake air supplied to the engine is small.
However, when the throttle valve is only slightly open or fully closed, such as during engine idling, the amount of intake air is adjusted by opening and closing the flow control valve to adjust the supply of assist air. Accordingly, when assist air is continuously supplied to the engine via the fully-opened assist air passage as described above, excess air is supplied to the engine, which causes overrunning of the engine.
On the other hand, as disclosed in Japanese Second (examined) Patent Publication No. 55-9555, there has been proposed a system wherein an air amount adjusting valve of a bypass air duct is controlled by means of a mechanical linkage which works in response to engine coolant temperature, so as to facilitate fuel atomization, particularly at a low engine temperature for effective engine start-up. Specifically, the opening of the air amount adjusting valve is increased as the engine temperature is reduced, and is decreased as the engine temperature increases so that the amount of air supplied via the bypass air duct is inversely proportional to the engine temperature.
When this prior art system is combined with the foregoing timed AMS, the combined system facilitates the warming up of the engine by supplying a large amount of assist air to the cold engine, and prevents overrunning of the engine by reducing the opening of the flow control valve as the engine temperature increases, so as to reduce the amount of assist air supplied to the engine.
However, in such a combined system, since the opening of the flow control valve varies with engine temperature not only during the engine idling but also during high-load operation, the combined system is not effective during high-load operation.
Specifically, during high-load operation, the throttle valve is opened wide and the amount of intake air sucked into the engine is large, thus the intake pressure downstream of the throttle valve is increased. Accordingly, the pressure differential between the intake pressure and atmospheric pressure is reduced. As a result, the amount of assist air is also reduced, thus reducing the atomization of the injected fuel. In the foregoing combined system, the opening of the flow control valve is reduced in response to an increase in engine temperature. Accordingly, during high-load operation, the flow control valve works as a throttling against the assist air to generate a pressure loss in the assist air passage so that the assist air supply is further reduced. This leads to poor atomization of the injected fuel so that the timed AMS does not work effectively during high-load operation.